trailrat is about right.
Diversity will apply to a cooker, so you will use the first 10 amps, plus 30% of the remainder to get your cable / overcurrent device size.
11.4kW is 49 amps.
So 10 A plus 30% of 39A = 10A + 13A = 23A
Your cable will need to be able to carry 23A continuous.
Current carrying capacity of 6mm T+E cable is around 47A if buried in plaster, with no insulation around it. If bunched with other cables, its capacity drops, so assume it has a little insulation, or is bunched, so it will now have a current carrying capacity of 32A.
Which is perfect for your existing installation, as your new cooker will be a direct connection to the existing installation.
There’s a caveat that the Manufacturers installation instructions must be observed. They may require the use of a RCD to protect the cooker, so, in your case, having a fuse as your overcurrent device, I doubt if you have RCD protection on that circuit.
So simplistically, your existing installation will be fine, if the manufacturers instructions do not want RCD protection.